1. Field of the Invention
The invention relates to a digital optical switch, in which two asymmmetric waveguides are arranged to run at least closely adjacent over a short path length, and are constructed to be electro-optically controllable in a central region with respect to their adiabatic coupling and also are provided in the central region with a grid arrangement which affects the waveguide eigenmodes.
Digital optical switches--termed "DOS" for short hereinbelow--are required particularly in transparent optical nets. They--as well as other components--should make through optical transmission paths possible, in which optical fibers, as a transmission medium in the wavelength region of about 1.3-1.6 .mu.m, are to provide a transmission bandwidth totaling about 50 THz and usable simultaneously for several channels. The optical switches, to be used for example in multiplexers/demultiplexers and also to be grouped together to form switch matrices in switching or distribution centers, are to be substantially independent of polarization and wavelength, and are to have a short overall length as integrable components and moreover low losses and the lowest possible crosstalk.
2. Discussion of Relevant Art
A digital optical switch of the kind stated at the beginning is known from WO 93/17363, to the extent that there, the central region is formed by a bimodal waveguide, which effects mode conversion, and which has for this purpose periodic cross section changes and a single electrode, and furthermore can be provided with asymmetric Y-branches on the output side and/or input side (cf. in the citation, FIG. 1 an also Specification page 13, lines 4-12). The function of this known optical switch is based on permitting, or not permitting, a conversion of the null and first order modes to take place. In this manner, in/out switches or direction couplers can be embodied.
Another kind of digital optical switch is known from "Applied Physics Letters" 51(16), October 1987, pages 1230-1232. According to this, the asymmetric waveguides which cross each other in a four-gate digital optical switch can be constructed with a different width between the input and the crossing region and with the same width between the crossing region and the output, but made opto-electronically controllable with respect to their refractive index. So that the adiabatic coupling is not interrupted, the electrodes serving for electro-optic control can be shaped so that the changes are gradual in the electrical fields which are acting.
The function of a DOS in X-structure, which is based on the principle of adiabatic asymmetric Y-branching, thus consists in that the light waves which are conducted into the input region of a light waveguide reach, in the central region, only that waveguide branch in which the same conditions prevail for the propagation of this light wave. If then the propagation properties of the waveguide branches leading to the outputs are thus respectively set to the propagation properties of the one or of the other input waveguide branch, the light waves fed into the X-DOS can--even simultaneously--be directed to one or other output. It furthermore has excellent properties as regards polarization and wavelength independence, but however does not fulfill, as it stands, the particularly high requirements regarding the crosstalk permissible in transparent optical nets. A crosstalk damping of 20 dB, under favorable circumstances 30 dB, is not sufficient when an optical signal has to run through for example 100 cascaded stages. For this, 40 dB per stage is required, and indeed in the circumstances arising in the least favorable case.
The following is to be taken into account for other methods, e.g., "total internal reflection" (TIR) and "two mode interference" (TMI), with which the crosstalk damping of a DOS could be improved:
In both these methods, the desired polarization and wavelength independence are in any case not already provided. PA0 According to the TIR method, the preconditions of short overall length and particularly of low crosstalk are indeed fulfilled, but comparatively high losses have to be considered, if indeed a material were available at all which makes possible the refractive index difference .DELTA.n.sub.EO of about 0.3 which is required in these methods. PA0 The TMI method, in contrast to this, can be embodied with available materials, and low losses can also be expected, but it requires a large overall length and a precise power distribution which is dependent on close tolerances which have to be maintained in the manufacturing process.
Surprisingly, another kind of starting point offers a solution for the technical problem on which the invention is based. In [German Patent] DE 44 04 777 A1 of Feb. 10, 1994, a control element for acting on light beams in optical waveguides is disclosed, concerning which a report was already given on the occasion of the conference "Integrated Photonics Research 1994", Feb. 17-19, 1994, in San Francisco, Calif. (U.S.A.) (see H.-P. Nolting, M. Gravert: "Theoretical Investigation of a Tunable, Narrowband, Electro-Optical Filter with Low Crosstalk and a Large Number of Optical Channels", Technical Digest Series, Vol. 3 (Optical Society of America, Washington, DC, 1994), pages 290-292, respectively FB 3-1 to FB 3-3). This control element provides for individual patterns, which respectively are associated with a given wavelength within a spectral region. The control element can thus be set to given operating conditions and can carry out a mode conversion, e.g. for light rays of a given wavelength.